Neural mechanisms which intervene between the stimulation of dopamine receptors and the resulting behavior will be studied. One aspect examines how neurotransmitters microinjected into the nucleus accumbens through chronically implanted cannulae modulate the circling behavior of rats with unilateral 6-hydroxydopamine lesions of the caudate nucleus. Another aspect examines the neurochemical changes with accompany electrolytic lesions of the nucleus accumbens by measuring catecholamines, serotonin, choline acetyltransferase and glutamate decarboxylase in discrete brain regions after such lesions. The neurochemical mechanisms of a profound hyperactivity which follows electrolytic lesions of the nucleus accumbens will be examined by investigating if more selective neurotoxic lesions with 5,7 dihydroxytryptamine and kainic acid mimic the behavioral effects of electrolytic lesions. The pharmacological basis of the marked sedative properties demonstrated by apomorphine in this model will be examined by the use of dopamine agonists and antagonists. The location of these receptors is examined by intracranial injection experiments and 6-hydroxydopamine lesions. An unorthodox apomorphine-induced circling we find after small kainic acid-induced lesions of the striatum is further studied to characterize the anatomical basis of the effect and some of its pharmacological properties.